Dendritic Highly Branched Polymers

There is an ongoing effort to synthesize different, well-defined polymer architectures. A considerable thrust in this direction has been the synthesis of dendritic polymers [Bosnian et al., 

Two types of dendritic polymers have been developed hyperbranched polymers and dendrimers. 

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DSM has commercialized new dendritic (highly branched) polymers, under the name Hyhrane. Related to the original dendrimer. Astramol, these performance additives are described as hyperbranched . They do not require such a perfect structure as a dendrimer, but they generally retain many of the properties characteristic of these materials. 

Whereas the well-characterized, perfect (or nearly so) structures of dendritic macromolecules, constructed in discrete stepwise procedures have been described in the preceding chapters, this Chapter reports on the related, less than perfect, hyperbranched polymers, which are synthesized by means of a direct, one-step polycondensation of A B monomers, where x > 2. Flory s prediction and subsequent demonstration 1,2 that A B monomers generate highly branched polymers heralded advances in the creation of idealized dendritic systems thus the desire for simpler, and in most cases more economical, (one-step) procedures to the hyperbranched relatives became more attractive. 

Dendritic architectures are highly branched polymers with tree like branching having an overall spherical or ellipsoidal shape and are known as additives having peripheral functional groups. These macromolecules consist of three subsets namely dendrimers, dendrigraft polymers and hyperbranched polymers (Figrrre 1). 

Most polymers have chain-Uke molecular geometry and are referred to as linear polymers. However, other geometries such as highly branched polymers, comb-like pol5mters, cross-linked polymers, and dendritic polymers also exist. 

Molecular encapsulation in dendritic and highly branched polymers has received increasing attention recently. It was shown for dendrimers that the site isolation of a fimaional core unit in a dendritic scaffold is of great interest with respect to optical properties, catalysis, and other future applications. Detailed studies on dendrimers have revealed that at some critical dendrimer generation, the core is encapsulated by the sterically aowded and densely packed highly branched architecture. Within this context, Frechet et al introduced l-(N,N-dimethylamino)-4-nitrobenzene as a solvatochromic chromophore at the focal point of a poly(benzyl ether) dendrimer. In other works, manganese and zinc porphyrins... 

Astruc and coworkers have reported the synthesis of highly branched polymers coordinated to cyclopentadienyliron and pentamethylcyclopentadienyl-ruthe-niiun cations." The catalytic and sensing ability of star polymers and dendrimers has also been reviewed. Multifunctional core molecules suitable for the synthesis of star and dendritic materials were synthesized by peralkylation or aUylation of methyl-substituted arene complexes of cyclopentadienyliron." The benzylic protons on these complexed arenes are acidic, which permits their facile alkylation. These branched polymers contained cationic cyclopentadienyliron moieties at the core and/or the periphery. The synthesis of a water-soluble metallodendrimers... 

Star and hyperbranched (dendritic) polymers have attracted increasing attention in organic, supermolecular, and polymer chemistry as well as coordination chemistry because of their specific structures and characteristics [1-5]. For the preparation of these highly branched polymers, two kinds of synthetic methods have been developed a one-step pol)nnerization and a stepwise method. By the one-step method, various star and hyperbranched polymers with many structural and functional group variations have been prepared [6-10]. On the other hand, the stepwise method is quite useful, especially for the synthesis of dendritic polymers. Both divergent and convergent s)mthetic approaches have been employed [2,11]. 

Physical properties of highly branched polymers are different from their linear analogs and have many potential applications in medicine and nanoengineering. Dendritic polymers with monodispersed composition and weU-defined tree-like structures are even attractive. Efficient reactions are required, and thiol-ene was used by Hawker and coworkers to prepare dendrimers starting from a tris-alkene core 2,4,6-triallyloxy-l,3,5-triazine. 1-Thioglycerol was used to introduce two hydroxy groups via UV-irradiated thiol-ene reaction, which was carried out at room temperature for 30 min. Due to the efficient nature of thiol-ene chemistry and less byproducts, hydroxy-terminated dendrimers... 

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